1. Field of the Invention
This invention relates generally to wire matrix printing elements, and more particularly to serial wire matrix printing elements in which a piezoelectric crystal is used to actuate the printing wire or missile.
2. Prior Art
There have been many prior art proposals for actuation of serial wire matrix printhead devices. One conventional technique has been electromagnetic actuation wherein the printheads are actuated by electromagnetic phenomena to perform the printing function. This particular technique while having many advantages does have several serious limitations. One limitation is the inductance of the coil necessary to create the driving magnetic flux field which requires a reasonably long time for the current to build to its required value for actuation and thus limits the speed at which the device can operate. Further, due to the relative large amount of power consumed, thermal considerations typically play a very large part in the design of the device. Most printhead designs of this type require some means to sense the temperature of the printhead and to slow down, stop or partially disable the device if the heat rises above a preselected value, to prevent damage to the head. This also results either in a slowing down of the printing speed or a degradation in the quality of the printing output. Also, this arrangement, due to the large heat build up, prevents there being a large number of actuators tightly packed which also degrades the quality of the printed character. Also, because of packaging considerations, the total package is relatively large and bulky.
There has been suggested the use of printheads using piezoelectric actuators to overcome the problems enumerated above. The presently available types of designs of piezoelectric actuators include the multi-laminate benders (typically referred to as bimorph), and stacked multi-actuators.
The bimorph type configuration does provide the large displacement required for actuation of the print wire but has significant reliability problems as well as speed limitations and lacks the necessary precision in control of the required movement. U.S Pat. No. 4,035,671 as well as IBM Technical Disclosure Bulletin, Volume 26, No. 1, June 1983 at Pages 49 and 50 and IBM Technical Disclosure Bulletin, Volume 26, No. 8, January, 1984 at Page 3984 all disclose various aspects of bimorph type of piezoelectric actuation.
The stacked multi-layer actuated design provides quick response and low driving voltage and large generator forces. However, there is a very small displacement which requires some form of displacement amplification.
U.S. Pat. Nos. 3,473,466; 3,649,857; 4,783,610; 4,589,786; and U.S. Pat. No. 4,547,086 all disclose various forms of a mechanical amplification of movement of a piezoelectric crystal for amplification. U.S. Pat. No. 3,970,184 shows hydraulic amplification of piezoelectric movement and U.S. Pat. No. 4,193,703 and IBM Technical Disclosure Bulletin, Volume 26, November, 1977, Page 2263 shows a buckling beam type amplification of piezoelectric crystal actuation for printing elements. In this buckling beam type actuation, the piezoelectric crystals are energized which in turn actuates a beam which is in a flexed condition pushing it to a more flexed condition which operates against the printing wire. This configuration relies on the inherent strength and rigidity of the buckling beam to push the print wire. All of these devices have several limitations which limit their effective use.